HVAC unit with hot gas reheat

ABSTRACT

The present disclosure is directed to a single compressor HVAC system with hot gas reheat. The system includes a single compressor, a pair of condensers, a reheat heat exchanger, an evaporator, and an expansion device. Within the system, the refrigerant exiting the compressor is separated into two portions. In the cooling mode, the first and second portions of the refrigerant are directed from the compressor through the two condensers in parallel. In the reheat mode, the first portion of the refrigerant is directed through the first condenser, while the second portion of the refrigerant is directed through the reheat heat exchanger. The system also may include a head pressure control device that is designed to maintain the compressor discharge pressure within a desired range by adjusting the condenser fan speed.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.13/370,098, entitled “HVAC UNIT WITH HOT GAS REHEAT,” filed Feb. 9,2012, which claims priority from and the benefit of U.S. ProvisionalApplication Ser. No. 61/441,964, entitled “HVAC UNIT WITH HOT GASREHEAT,” filed Feb. 11, 2011, which are hereby incorporated byreference.

BACKGROUND

The invention relates generally to heating, ventilating, and airconditioning systems (HVAC), and more particularly to HVAC systemsemploying hot gas reheat for humidity control.

A wide range of applications exists for HVAC systems. For example,residential, light commercial, commercial, and industrial systems areused to control temperatures and air quality in residences andbuildings. Very generally, HVAC systems may include circulating a fluid,such as a refrigerant, through a closed loop between an evaporator wherethe fluid absorbs heat and a condenser where the fluid releases heat.The fluid flowing within the closed loop is generally formulated toundergo phase changes within the normal operating temperatures andpressures of the system so that considerable quantities of heat can beexchanged by virtue of the latent heat of vaporization of the fluid.

HVAC units, such as air handlers, heat pumps, and air conditioningunits, are used to provide heated, cooled, and/or dehumidified air toconditioned environments. Dehumidification may be desired on days whenthe temperature is cool and there is a high humidity level, such asdamp, rainy, spring and fall days. Further, certain spaces, such asrefrigerator cases, locker rooms, food production lines, art galleries,and museums, may benefit from a low humidity environment. Accordingly,it may be desirable to operate an HVAC system in a reheatdehumidification mode.

In the reheat mode, humidity may be removed by cooling and thenreheating air that is provided to the conditioned space. The air can bereheated using electric or gas heat; however, these heating methods maybe costly. The air also can be reheated by passing the air over a reheatheat exchanger that circulates heated refrigerant from the closed loopof the HVAC system. However, when the refrigerant is circulated throughthe reheat heat exchanger, it may be difficult to maintain a consistentrefrigerant charge level within the HVAC system. Further, additionalequipment, such as a second compressor, may be desired when employing areheat heat exchanger within a HVAC system.

DRAWINGS

FIG. 1 is an illustration of an embodiment of a commercial or industrialHVAC system that employs a hot gas reheat system, in accordance with thepresent techniques.

FIG. 2 is a perspective of the HVAC unit shown in FIG. 1 with a portionof the cabinet removed to reveal internal components.

FIG. 3 is a schematic diagram of the HVAC unit shown in FIG. 1 operatingin the cooling mode.

FIG. 4 is a schematic diagram of the HVAC unit shown in FIG. 1 operatingin the reheat mode.

FIG. 5 is a flowchart depicting a method for regulating pressure whenoperating in the reheat mode.

DETAILED DESCRIPTION

The present disclosure is directed to an HVAC system that employs anovel hot gas reheat configuration to provide humidity control. The HVACsystem includes a single compressor, at least two condensers, a reheatheat exchanger, an evaporator, and an expansion device. The HVAC systemis designed to operate in a cooling mode, which provides cooled anddehumidified air to the conditioned space, and in a reheat mode thatprovides dehumidified air to the conditioned space when little or nosensible cooling is desired.

The HVAC system includes a closed refrigeration loop that circulates therefrigerant through the system. Within the system, the refrigerantexiting the compressor is separated into two portions. In the coolingmode, the first and second portions of the refrigerant are directed fromthe compressor through the two condensers in parallel. Approximatelyhalf of the refrigerant exiting the compressor is condensed in eachcondenser. However, in other embodiments, the amount of refrigerantcondensed in each condenser may vary. The condensed refrigerant from thetwo condensers is then combined and directed through an expansion deviceand an evaporator to produce cooled air that is provided to theconditioned space. In the reheat mode, the first portion of therefrigerant is directed through the first condenser, while the secondportion of the refrigerant is directed through the reheat heat exchangerto heat air cooled by the evaporator. The refrigerant flows through thereheat heat exchanger and the first condenser in parallel, and norefrigerant is directed through the second condenser. The refrigerantexiting the first condenser and the reheat heat exchanger is thencombined and directed through the evaporator.

The second condenser, which is used in the cooling mode, and the reheatheat exchanger, which is used in the reheat mode, may have approximatelyequal interior volumes for receiving refrigerant. Accordingly, in boththe cooling mode and the reheat mode, the refrigerant may be circulatedwithin a closed refrigeration loop of approximately the same volume,which may allow the flow of refrigerant to be balanced in the HVACsystem during both the cooling and reheat modes. However, in otherembodiments, the relative volumes of the second condenser and the reheatheat exchanger may vary depending on factors such as system designcharacteristics and the amount of reheat that is desired, among others.Regardless of the relative volumes, a single compressor can be used inthe HVAC system to provide both cooling and dehumidification control. Incertain embodiments, the HVAC system also may include a head pressurecontrol device that is designed to maintain the compressor dischargepressure within a desired range by adjusting the condenser fan speed.Maintaining the compressor discharge pressure may be particularlybeneficial when the HVAC system is operating in the reheat mode toensure that there is a sufficient flow of refrigerant through the reheatheat exchanger.

FIG. 1 illustrates an exemplary application, in this case a heating,ventilating, and air conditioning (HVAC) system for buildingenvironmental management that includes one or more HVAC units employinghot gas reheat. A building 10 is cooled, heated, and dehumidified by asystem that includes an HVAC unit 12 and a boiler 14. As shown, HVACunit 12 is disposed on the roof of building 10 and boiler 14 is locatedin the basement; however, the HVAC unit and boiler may be located inother equipment rooms or areas next to the building.

HVAC unit 12 is an air-cooled device that implements a refrigerationcycle to provide cooled and dehumidified air to building 10. HVAC unit12 may be a stand-alone unit or may be part of a single package unitcontaining other equipment, such as a blower, integrated air handler,and/or auxiliary heating unit. In certain embodiments, the HVAC unitalso may provide heating for the building. For example, in certainembodiments, the HVAC unit 12 may provide electric or gas heat. In theseembodiments, the boiler may not be required. However, in otherembodiments, the HVAC unit may provide cooling while the boiler providesheating.

Boiler 14 is a closed vessel that includes a furnace to heat water. Thewater from boiler 14 is circulated through building 10 by water conduits16. Water conduits 16 are routed to air handlers 18, located onindividual floors and within sections of building 10. Air handlers 18include heat exchangers that circulate hot water from boiler 14 toprovide heated air. Air handlers 18 also may provide the cooled air thefrom HVAC unit 12 to areas within building 10. Ductwork 20, coupled toair handlers 18, may receive air from an outside intake (not shown) andis adapted to distribute air between the air handlers. Fans, within airhandlers 18, direct the conditioned air to environments within building10, such as rooms, apartments, or offices, to maintain the environmentsat a designated temperature.

A control device, shown here as including a thermostat 22, may be usedto designate the temperature of the conditioned air. Control device 22also may be used to control the flow of air to and from air handlers 18.Other devices may, of course, be included in the system, such as controlvalves that regulate the flow of water and pressure and/or temperaturetransducers or switches that sense the temperatures and pressures of thewater, the air, and so forth. Moreover, control devices may includecomputer systems that are integrated with or separate from otherbuilding control or monitoring systems, and even systems that are remotefrom the building.

FIG. 2 depicts HVAC unit 12, shown here as an air-conditioning packageunit, with a portion of the external covering removed to show theinternal components. As a single package unit, the HVAC unit may includean independent refrigeration circuit and components that are tested,charged, wired, piped, and ready for installation. Of course, in otherembodiments, the HVAC unit may be a standalone unit that may beconnected to one or more external refrigeration circuits and associatedequipment. The HVAC unit may provide a variety of heating and/or coolingfunctions, such as cooling and dehumidification with reheat, or coolingand dehumidification with reheat and with electric or gas heat, amongothers.

A cabinet 24 encloses unit 12 to provide structural support and protectthe internal components from environmental and other contaminants.According to certain embodiments, the cabinet may be constructed ofgalvanized steel and insulated with insulation. Of course, in otherembodiments, the cabinet may be constructed of any suitable material.Rails 26 are adjoined to the bottom perimeter of cabinet 24 and providea solid foundation for unit 12. In certain embodiments, the rails mayprovide access for a three-way forklift and overhead rigging.

HVAC unit 12 includes four heat exchangers, including condensers 28, 30,an evaporator 32, and a reheat heat exchanger 34, that are in fluidcommunication with the refrigeration circuit. Tubes within the heatexchangers may circulate refrigerant, such as R-410A, or any othersuitable heat transfer fluid through the heat exchangers. The tubes maybe of various types, such as multichannel tubes, conventional copper oraluminum tubing and fins, and so forth. Together, the heat exchangersmay implement a thermal cycle in which the refrigerant undergoes phasechanges and/or temperature changes as it flows through the heatexchangers to produce heated and/or cooled air. For example, condensers28 and 30 may transfer heat from the refrigerant to ambient air to coolthe refrigerant. In another example, evaporator 32 may absorb heat froman air stream to produce cooled air that can be provided to theconditioned space. When HVAC unit 12 is operating in the reheat mode,the cooled air from evaporator 32 may be directed over reheat heatexchanger 34 to heat the air to the desired temperature for theconditioned space.

Evaporator 32 and reheat heat exchanger 34 are located within acompartment 36 that separates evaporator 32 and reheat heat exchanger 34from condensers 28 and 30. One or more fans 38 draw air from theenvironment through condensers 28 and 30. According to certainembodiments, condensers 28 and 30 may be separate and independent heatexchangers. However, in other embodiments, condensers 28 and 30 may beindependent coils included within the same heat exchanger. As theambient air flows across condensers 28 and 30, the ambient air absorbsheat from the refrigerant within the condensers 28 and 30 to condensethe refrigerant. The heated ambient air is then released back to theenvironment.

The condensed refrigerant can then be provided to evaporator 32. Ablower assembly 40 draws air through evaporator 32 and reheat heatexchanger 34 to provide cooled and/or dehumidified air to theconditioned space. In the cooling mode, reheat heat exchanger 34 isinactive and accordingly, the cooled air from evaporator 32 passesthrough reheat heat exchanger 34 without substantial heating or cooling.In the reheat mode, reheat heat exchanger 34 circulates high temperaturerefrigerant from a compressor 42. Accordingly, in the reheat mode, thecooled air from evaporator 32 can be heated as the air passes throughreheat heat exchanger 34. The cooled and/or dehumidified air may bedirected to the conditioned space by ductwork 20 (FIG. 1). Further, thehigh temperature refrigerant flowing through reheat heat exchanger 34may be condensed as the refrigerant transfers heat to the cooled airfrom evaporator 32.

HVAC unit 12 also may include other equipment for implementing thethermal cycle. Compressor 42 reduces the volume available for therefrigerant, consequently, increasing the pressure and temperature ofthe refrigerant before the refrigerant enters condensers 28 and 30 andreheat heat exchanger 34. The compressor 42 may be any suitable type ofcompressor, such as a scroll compressor, screw compressor, centrifugalcompressor, rotary compressor, or reciprocating compressor. As may beappreciated, additional equipment and devices may, of course, beincluded in the HVAC unit. For example, the unit may include one or moreair filters, a filter drier, a drain pan, a disconnect switch,economizers, pressure switches, phase monitors, and humidity sensors,among others.

HVAC unit 12 may receive power through a terminal block 44. For example,a high voltage power source may be connected to terminal block 44 topower the equipment. The operation of unit 12 may be governed by acontroller 46 that can include one or more control boards. According tocertain embodiments, controller 46 may include a primary control boardthat governs general operation of HVAC unit 12 and a reheat controlboard that governs the reheat aspects of HVAC unit 12. Controller 46 mayinclude control circuitry connected to a thermostat, humidistat,sensors, and/or alarms, among others. According to certain embodiments,controller 46 may be communicatively coupled to thermostat 22 (FIG. 1)to receive a desired temperature and/or humidity for the conditionedair. The controller 46 may be configured to control operation of theequipment, provide alarms, and monitor safety switches. Wiring 48 mayconnect control board 46 and terminal block 44 to the equipment of HVACunit 12.

FIGS. 3 and 4 are schematic diagrams that depict the flow of refrigerantthrough HVAC unit 12. In particular, FIG. 3 shows HVAC unit 12 operatingin a cooling mode, and FIG. 4 shows HVAC unit 12 operating in a reheatmode. The cooling mode can be employed to provide cooled air to aconditioned space, while the reheat mode can be employed to providedehumidified air to the conditioned space when additional cooling of theair is not desired. For example, on days when the ambient temperature isrelatively low and the humidity is high, the reheat mode may be employedto provide dehumidified air at a comfortable temperature.

As shown in FIG. 3, refrigerant flows through HVAC unit 12 within aclosed refrigeration loop 50. In the cooling mode, refrigerant flowsthrough evaporator 32, compressor 42, and condensers 28 and 30. Blowerassembly 40 draws air 52, generally represented by arrows, throughevaporator 32. As the air 52 flows across evaporator 32, the refrigerantflowing through evaporator 32 absorbs heat from the air to cool the air.The cooled air may then be provided to the conditioned space throughductwork 20. As the air is cooled, moisture also may be removed from theair to dehumidify the air. For example, as the air flows across heatexchanger tubes of evaporator 32, moisture within the air may condenseon the tubes and may be directed to a drain.

Blower assembly 40 also may draw the air 52 across reheat heat exchanger34, which is inactive in the cooling mode. Reheat heat exchanger 34 isdisposed generally downstream of evaporator 32 with respect to the airflow, and accordingly, the cooled air exiting evaporator 32 may flowthrough reheat heat exchanger 34. However, in the cooling mode, reheatheat exchanger 34 contains little or no refrigerant, and accordingly, nosubstantial heating or cooling occurs as the air 52 flows through reheatheat exchanger in the cooling mode.

As the air 52 flows through evaporator 32, the air transfers heat to therefrigerant flowing within evaporator 32. As the refrigerant is heated,at least a portion of, or a large portion of, the refrigerant mayevaporate into a vapor. The heated refrigerant exiting evaporator 32then flows through connection points 54 and 56 to enter the suction sideof compressor 42. Compressor 42 reduces the volume available for therefrigerant vapor, consequently, increasing the pressure and temperatureof the refrigerant.

The refrigerant exits the discharge side of compressor 42 as a highpressure and temperature vapor that flows to a connection point 58. Atconnection point 58, the refrigerant is split into two separateportions. In particular, a first portion is directed to condenser 30,and a second portion is directed to a three-way valve 60. In the coolingmode, the three-way valve 60 is in the cooling position 61, whichdirects the refrigerant through connection point 62 to condenser 28.Three-way valve 60 is located downstream of connection point 58 andupstream of condenser 28, which ensures that condenser 30 receives asteady flow of refrigerant in both the cooling mode and the reheat mode.

In summary, in the cooling mode, the refrigerant is separated into twoportions, with each portion flowing through a separate condenser 28 or30 in parallel. According to certain embodiments, condensers 28 and 30may be of approximately equal volume and/or size, allowing approximatelyhalf of the refrigerant by volume to be directed through each condenser28 and 30. Further, the use of two separate condensers 28 and 30 in thecooling mode may be designed to maximize the surface area that isavailable for heat transfer in the cooling mode.

One or more fans 38, which are driven by one or more motors 66, draw air64 across condensers 28 and 30 to cool the refrigerant flowing withincondensers 28 and 30. According to certain embodiments, motor 66 may becontrolled by a variable speed drive (VSD) or variable frequency drive(VFD) that can adjust the speed of the motor 66, and thereby adjust thespeed of the fans 38. Fans 38 may push or pull air across heat exchangertubes of condensers 28 and 30. As the air 64 flows across tubes ofcondensers 28 and 30, heat transfers from the refrigerant vapor to theair, producing heated air and causing the refrigerant vapor to condenseinto a liquid. The refrigerant exiting condenser 28 then flows through acheck valve 68 to a connection point 70 where the refrigerant iscombined with the refrigerant exiting condenser 30. The check valve 68may be designed to allow unidirectional flow within the closedrefrigeration loop 50 in the direction from condenser 28 to connectionpoint 70. In other words, check valve 68 may impede the flow ofrefrigerant from connection point 70 into condenser 28.

The condensed refrigerant from condensers 28 and 30 may then flowthrough a connection point 72. In the cooling mode, a check valve 74inhibits the flow of refrigerant from connection point 72 into a reheatcircuit 76 that can be employed in the reheat mode to heat air exitingevaporator 32. Accordingly, the refrigerant flows from connection point72 to an expansion device 78, where the refrigerant expands to become alow pressure and temperature liquid. In certain embodiments, some vaporalso may be present after expansion in the expansion device. Expansiondevice 78 may be a thermal expansion valve (TXV); however, according toother embodiments, the expansion device may be an electromechanicalvalve, an orifice, or a capillary tube, among others. Further, in otherembodiments, multiple expansion devices 78 may be employed. For example,in certain embodiments, the refrigerant exiting condenser 28 may beexpanded in one expansion device, while the refrigerant exitingcondenser 30 is expanded in another expansion device. In theseembodiments, the refrigerant may be combined downstream of the expansiondevices and upstream of evaporator 32. From expansion device 78, therefrigerant then enters evaporator 32, where the low temperature andpressure refrigerant may then again absorb heat from the air 52.

The operation of HVAC unit 12 can be governed by controller 46.Controller 46 can transmit control signals to compressor 42 (e.g., to amotor that drives the compressor) and to three-way valve 60 to regulateoperation of HVAC unit 12. Although not shown, controller 46 also may beelectrically coupled to blower assembly 40 and/or motor 66. Controller46 can receive input from thermostat 22, and sensors 80 and 82, and mayuse information received from these devices to determine when to switchthe HVAC unit 12 between the cooling mode and the reheat mode. Further,in other embodiments, the controller may receive inputs from local orremote command devices, computer systems and processors, and mechanical,electrical, and electromechanical devices that manually or automaticallyset a temperature and/or humidity related set point for the HVAC unit12.

Sensors 80 and 82 can detect the temperature and the humidity,respectively, within the conditioned space and may provide data and/orcontrol signals indicative of the temperature and humidity to controller46. Controller 46 may then compare the temperature and/or humidity datareceived from sensors 80 and 82 to a set point received from thermostat22. For example, the controller 46 may determine whether the sensedtemperature is higher than a temperature set point. If the sensedtemperature is higher than the set point, controller 46 may place theHVAC unit 12 in the cooling mode. In particular, the controller 46 mayenable compressor 42 and place three-way valve 60 in the coolingposition 61. In certain embodiments, controller 46 also may adjustoperation of the blower assembly 40 and the motor 66. In anotherexample, if the sensed temperature is below the temperature set point,controller 46 may then determine whether the sensed humidity is higherthan a humidity set point. If the sensed humidity is higher than the setpoint, and the space does not require cooling, controller 46 may placethe HVAC unit 12 in the reheat mode, as described further below withrespect to FIG. 4.

The controller 46 may execute hardware or software control algorithms togovern operation of the HVAC unit 12. According to certain embodiments,the controller 46 may include an analog to digital (A/D) converter, amicroprocessor, a non-volatile memory, and one or more interface boards.For example, in certain embodiments, the controller 46 can include aprimary control board that receives control signals and/or data fromcontroller 22 and temperature sensor 80. The primary control board maybe employed to govern operation of the compressor 42, as well as othersystem components. The controller 46 also can include a reheat controlboard that receives data and/or control signals from humidity sensor 82.According to certain embodiments, sensor 82 may be a dehumidistat. Thereheat control board may be employed to govern the position of three-wayvalve 60 and valves 84 and 86, as well as other system components.However, in other embodiments, the configuration of the controller 46may vary. Further, other devices may, of course, be included in thesystem, such as additional pressure and/or temperature transducers orswitches that sense temperatures and pressures of the refrigerant, theheat exchangers, the inlet and outlet air, and so forth.

According to certain embodiments, controller 46 may employ two differenttemperature set points to determine when to switch HVAC unit 12 betweenthe reheat mode and the cooling mode. For example, controller 46 may usea first temperature set point to determine when to place HVAC unit 12 inthe cooling mode when the humidity is low. If the sensed humidity isbelow the humidity set point, and the sensed temperature is above thefirst temperature set point, the controller 46 may operate HVAC unit 12in the cooling mode. Controller 46 may use a second temperature setpoint to determine when to place HVAC unit in the cooling mode when thehumidity is high. According to certain embodiments, the secondtemperature set point may be approximately 2 to 6 degrees higher thanthat the first temperature set point. If the sensed humidity is abovethe humidity set point and the temperature is above the secondtemperature set point, controller 46 may place HVAC unit 12 in thecooling mode. However, if the sensed humidity is above the humidity setpoint and the temperature is below the second temperature set point,controller 46 may operate the HVAC unit 12 in the reheat mode.

Controller 46 is also electrically coupled to valves 84 and 86 ofrefrigerant recovery circuits 88 and 90. Refrigerant recovery circuits88 and 90 can be employed to recover refrigerant from reheat heatexchanger 34 and condenser 28, respectively, when switching between thecooling mode and the reheat mode. For example, when switching from thecooling mode to the reheat mode, controller 46 may open valve 86 todirect refrigerant from condenser 28 through connection point 62 andvalve 86 to connection point 56 where the refrigerant may be directed tothe suction side of compressor 42. When switching from the reheat modeto the cooling mode, controller 46 may open valve 84 to drainrefrigerant from reheat heat exchanger 34 through connection point 89,and valve 84 to connection point 54 where the refrigerant may bedirected to the suction side of compressor 42. Both recovery circuits 88and 90 are connected to the suction side of compressor 42 to drawrefrigerant from the refrigerant recovery circuits 88 and 90 back to thecompressor 42.

According to certain embodiments, refrigerant recovery circuits 88 and90 are designed to allow refrigerant from the inactive reheat heatexchanger 34 or condenser 28 to return to the compressor 42. The returnof refrigerant to the compressor 42 may ensure that most, or all, of therefrigerant is circulated through the compressor 42 in both the coolingmode and the reheat mode. Accordingly, in the cooling mode shown in FIG.3 (when three-way valve 60 is in cooling position 61), valve 84 may beopen while valve 86 is closed. In the reheat mode shown in FIG. 4 (whenthree-way valve 60 is in reheat position 94) valve 86 may be open whilevalve 84 is closed.

Controller 46 may cycle valve 84 or 86 on and off or may leave valve 84or 86 open to allow refrigerant from the inactive reheat heat exchanger34 or condenser 28 to return to the compressor 42. For example, incertain embodiments, controller 46 may close valve 84 or 86 after a setamount of time. However, in other embodiments, controller 46 may leavevalve 84 or 86 open until switching to the other mode of operation. Forexample, in these embodiments, controller 46 may close valve 84 whenswitching to the reheat mode, and may close valve 86 when switching tothe cooling mode.

HVAC unit 12 also includes a control device 92 that may be employed toregulate pressure within closed refrigeration loop 50. According tocertain embodiments, control device 92 may be designed to ensure that aminimum pressure differential is maintained across expansion device 78.Control device 92 is coupled to a pressure transducer 93 that detectsthe discharge pressure of the compressor 42. As shown, pressuretransducer 93 is disposed in the closed refrigeration loop 50 betweencompressor 42 and connection point 58. However, in other embodiments,pressure transducer 93 may be disposed in other suitable locations onthe high-pressure side of refrigeration loop 50. For example, thepressure transducer may be located between compressor 42 and three-wayvalve 60. In another example, the pressure transducer may be locatedbetween check valve 68 and expansion device 78 or between condenser coil30 and expansion device 78. Further, in other embodiments, any suitabletype of pressure sensor may be used. For example, in certainembodiments, the pressure sensor may include one or more pressureswitches and/or relays. Moreover, in certain embodiments, control device92 may be integrated with controller 46.

Control device 92 may receive data indicative of the discharge pressureand may adjust the speed of the condenser fan motor 66 to maintain thepressure within a desired range. For example, control device 92 maytransmit control signals to motor 66 to increase or decrease the fanspeed. Further, in certain embodiments, control device 92 may include aVSD or VFD that adjusts the speed of motor 66. In the cooling mode,control device 92 may be employed at low ambient temperatures to ensurethat an adequate pressure differential is maintained across expansiondevice 78. Control device 92 also may be employed to maintain sufficientflow of refrigerant through the reheat heat exchanger 34 when operatingin the reheat mode, as discussed below with respect to FIG. 4.

FIG. 4 depicts HVAC unit 12 operating in the reheat mode. As discussedabove, the reheat mode may be employed to provide dehumidification whenadditional cooling is not desired. For example, on days when the ambienttemperature is low but the humidity is high, it may be desirable toprovide dehumidified air that is not substantially reduced intemperature to avoid over cooling the space. High-pressure andtemperature refrigerant exits compressor 42 and is directed toconnection point 58 where the refrigerant is split into two portions. Asin the cooling mode shown in FIG. 3, the first portion flows fromconnection point 58 to condenser 30. The second portion flows throughthree-way valve 60, which is now located in the reheat position 94 todirect the refrigerant into reheat circuit 76. Accordingly, in thereheat mode, no refrigerant is directed into the condenser 28. Further,valve 86 can be opened by controller 46 to drain refrigerant fromcondenser 28 to the suction side of compressor 42 through refrigerantrecovery circuit 90.

Similar to the cooling mode, the first portion of refrigerant flowsthrough condenser 30 and transfers heat to the air 64 to condense therefrigerant. The condensed refrigerant then exits condenser 30 and flowsthrough connection points 70 and 72. Check valve 68 inhibits the flow ofrefrigerant from connection point 70 to condenser 28. Accordingly, therefrigerant exiting condenser 30 is directed through connection points70 and 72 to expansion device 78.

The second portion of refrigerant flows through three-way valve 60 andinto reheat circuit 76. Three-way valve 60 is located downstream ofconnection point 58 and upstream of condenser 28, which may ensure thatthere is sufficient flow of refrigerant through condenser 30 in thereheat mode. Within reheat circuit 76, the refrigerant, which isprimarily vapor, flows through connection point 89 to reheat heatexchanger 34. As the refrigerant flows through reheat heat exchanger 34,the refrigerant transfers heat to the air 52 exiting the evaporator 32.In other words, the high temperature refrigerant flowing through reheatheat exchanger 34 heats the air exiting evaporator 32. Accordingly, inthe reheat mode, the air 52 is first cooled and dehumidified as the airflows through evaporator 32. The cooled air is then reheated as the air52 flows through reheat heat exchanger 34. Accordingly, the dehumidifiedair can be provided to the conditioned space through ductwork 20.

As the refrigerant flows through reheat heat exchanger 34, therefrigerant transfers heat to the air 52 and the refrigerant iscondensed. According to certain embodiments, the refrigerant exitingreheat heat exchanger 34 may be condensed and/or subcooled. Therefrigerant then flows through check valve 74 to connection point 72,where the refrigerant is combined with the condensed refrigerant exitingcondenser 30. The refrigerant from connection point 72 is then directedthrough expansion device 78 and evaporator 32. From evaporator 32, therefrigerant returns to the compressor 42 where the process may beginagain.

In summary, in the reheat mode, refrigerant from compressor 42 isseparated into two separate portions that are directed through thecondenser 30 and the reheat heat exchanger 34 in parallel, whilecondenser 28 is inactive. According to certain embodiments, condensers28 and 30 and the reheat heat exchanger 34 may have approximately equalvolumes. Accordingly, in the reheat mode, the refrigerant is circulatedwithin a closed refrigeration loop of approximately the same volume asin the cooling mode, which may allow the flow of refrigerant to bebalanced in the HVAC system during both the cooling and reheat modes.However, in other embodiments, the relative volumes of the condensersand the reheat heat exchanger may vary depending on factors such assystem design characteristics and the amount of reheat desired, amongothers. In the reheat mode, refrigerant flows directly from compressor42 to reheat heat exchanger 34 (without flowing through a condenser 28or 30), which provides additional heating capacity for heating the airexiting evaporator 32 to the desired temperature. Moreover,substantially all the refrigerant exiting compressor 42 flows throughevaporator 32, which provides additional capacity for dehumidification.

Control device 92 may be employed in the reheat mode and/or in thecooling mode to maintain the pressure within the closed refrigerationloop 50. According to certain embodiments, control device 92 may be aprogrammable device that allows a user, manufacturer, or fieldtechnician, to set a target pressure range for the compressor dischargepressure sensed by pressure transducer 93. The pressure range may varydepending on factors such as the type of refrigerant employed. Further,the target pressure range may vary depending on factors such as theapplication of the HVAC unit 12 and the environment in which the HVACunit 12 is used, among others. The control device 92 may be employed inthe cooling mode and/or in the reheat mode to maintain the compressordischarge pressure within the specified pressure range. Further, it maybe beneficial to control the pressure when operating in the reheat modeto ensure that sufficient refrigerant flows through reheat heatexchanger 34 when ambient temperatures are low to ensure properdehumidification. According to certain embodiments, control device 92may be designed to regulate the pressure to maintain a near constantflow of refrigerant through reheat heat exchanger 34 as the ambienttemperature changes. Further, in certain embodiments, the pressure rangeselected for control device 92 may be designed to maintain a minimumpressure differential across expansion device 78.

FIG. 5 depicts a method 98 that may be employed to regulate the pressurewithin closed refrigeration loop 50 when HVAC unit 12 is operating inthe reheat mode. The method 98 may begin by determining (block 100) thecompressor discharge pressure. For example, pressure transducer 93 maydetect the compressor discharge pressure and provide signals indicativeof the pressure to control device 92. Control device 92 may thendetermine (block 102) whether the pressure is above or below thelow-pressure set point. According to certain embodiments, control device92 may include a storage that stores a low-pressure set point and ahigh-pressure set point, which define a target range of compressordischarge pressures. One or more hardware or software algorithms may beexecuted to determine whether the detected pressure is below thelow-pressure set point.

If the pressure is below the low-pressure set point, control device 92may then decrease (block 104) the fan speed. For example, control device92 may send a control signal to motor 66 to decrease the rotationalspeed of fan 38. According to certain embodiments, control device 92 maybe designed to linearly adjust the fan speed based on the detectedpressure. Further, in certain embodiments, control device 92 may bedesigned to decrease (block 104) the fan speed to below approximately200 revolutions per minute (RPMS). Decreasing the fan speed at lowambient temperatures may raise the temperature of the refrigerantflowing through condenser 30, which in turn, may increase the pressurewithin condenser 30 to promote the flow of refrigerant through reheatcircuit 76 and reheat heat exchanger 34.

If the pressure is not below the low-pressure set point, control device92 may then determine (block 106) whether the pressure is a above ahigh-pressure set point. For example, control device 92 may compare thedetected pressure to the high-pressure set point. One or more hardwareor software algorithms may be executed to determine whether the detectedpressure is above the high-pressure set point. If the pressure is abovethe high-pressure set point, control device 92 may increase (block 108)the fan speed. For example, the control device 92 may send a controlsignal to motor 66 to increase the rotational speed of fan 38. If thepressure is not above the high-pressure set point, control device 92 maymaintain (block 110) the present fan speed.

According to certain embodiments, control device 92 may be designed tocontinuously or periodically detect the pressure and adjust the fanspeed based on the pressure to maintain the discharge pressure withinthe desired range. Further, in certain embodiments, adjustments may bemade to the fan speed based on trends in the detected pressure.Moreover, in certain embodiments, control device 92 may be designed toiteratively adjust the fan speed based on the detected pressure tomaintain a discharge pressure that is in approximately the middle of thedesired pressure range.

While only certain features and embodiments of the invention have beenillustrated and described, many modifications and changes may occur tothose skilled in the art (e.g., variations in sizes, dimensions,structures, shapes and proportions of the various elements, values ofparameters (e.g., temperatures, pressures, etc.), mounting arrangements,use of materials, orientations, etc.) without materially departing fromthe novel teachings and advantages of the subject matter recited in theclaims. The order or sequence of any process or method steps may bevaried or re-sequenced according to alternative embodiments. It is,therefore, to be understood that the appended claims are intended tocover all such modifications and changes as fall within the true spiritof the invention. Furthermore, in an effort to provide a concisedescription of the exemplary embodiments, all features of an actualimplementation may not have been described (i.e., those unrelated to thepresently contemplated best mode of carrying out the invention, or thoseunrelated to enabling the claimed invention). It should be appreciatedthat in the development of any such actual implementation, as in anyengineering or design project, numerous implementation specificdecisions may be made. Such a development effort might be complex andtime consuming, but would nevertheless be a routine undertaking ofdesign, fabrication, and manufacture for those of ordinary skill havingthe benefit of this disclosure, without undue experimentation.

The invention claimed is:
 1. A system, comprising: a compressorconfigured to circulate a refrigerant through a refrigeration circuit; afirst condenser disposed along the refrigeration circuit, wherein thesystem is configured to direct a first portion of the refrigerant to thefirst condenser in a first operating mode of the system and in a secondoperating mode of the system, and wherein the first condenser isconfigured to transfer heat from the first portion of the refrigerant toambient; a second condenser disposed along the refrigeration circuit,wherein the system is configured to direct a second portion of therefrigerant to the second condenser in the first operating mode, whereinthe system is configured to direct the second portion of the refrigerantto bypass the second condenser in the second operating mode, and whereinthe second condenser is configured to transfer heat from the secondportion of the refrigerant to ambient; a reheat heat exchanger disposedalong the refrigeration circuit, wherein the system is configured todirect the second portion of the refrigerant to the reheat heatexchanger in the second operating mode, wherein the system is configuredto direct the second portion of the refrigerant to bypass the reheatheat exchanger in the first operating mode, and wherein the reheat heatexchanger is configured to transfer heat from the second portion of therefrigerant to a cooling fluid; and a pressure control system disposedon a high pressure side of the compressor, wherein the pressure controlsystem is configured to measure a discharge pressure of the refrigerantexiting the compressor and adjust a fan speed of the first condenserbased on the discharge pressure when the system is in the secondoperating mode.
 2. The system of claim 1, wherein the first condensercomprises one or more fans configured to direct ambient air over coilsof the first condenser such that heat transfers from the first portionof the refrigerant to the ambient air.
 3. The system of claim 2, whereinthe pressure control system is configured to adjust the fan speed of thefirst condenser by adjusting a speed of one or more motors configured tosupply power to the one or more fans.
 4. The system of claim 1, whereinthe pressure control system comprises a controller and a pressuresensor.
 5. The system of claim 4, wherein the pressure control systemcomprises a variable speed device, a variable frequency device, or both,configured to adjust a speed of a motor configured to supply power toone or more fans of the first condenser.
 6. The system of claim 1,wherein the pressure control system is configured to maintain thedischarge pressure of the compressor within a predetermined range byadjusting the fan speed of the first condenser.